Various types of battery charger circuits are known in the art. For example, the two most common types of battery charger circuits are linear and high frequency (also known as switched mode) battery charger circuits. Both types of battery charger circuits are known to have advantages and disadvantages.
Linear charger circuits normally include a transformer, a rectifier and a current regulating element. The primary of the transformer is normally connected to an external 120 volt AC power supply. The transformer steps down the voltage from the 120 volt AC power supply to an appropriate voltage for charging a battery, for example 12 volts AC. A rectifier, such as, a full wave rectifier, converts the stepped down AC voltage on the secondary winding of the transformer to a DC charging voltage. In some known linear charger circuits, a passive linear element, such as a resistor, is connected in series with the secondary winding of the transformer to limit the charging current provided to the battery. Linear charger circuits may also include a voltage regulator between the passive element and the secondary winding of the transformer to stabilize the output voltage. The charging current of such linear charger circuits is a linear function of the voltage of the 120 volt AC power supply source.
High frequency charger circuits are also known. An exemplary high frequency transformer is described in detail in U.S. Pat. No. 6,822,425, hereby incorporated by reference. In general, such high frequency charger circuits normally are connected to an external 120 volt AC power supply. The 120 volts AC from the 120 volt AC power supply is rectified, for example, by a full wave rectifier, to generate a DC voltage. The DC voltage is switched on and off by electronic switching circuitry to create a high frequency pulse train, for example, at frequencies from 10 KHz to 1 MHz, and applied to a high frequency transformer. The high frequency transformer steps down the voltage to an appropriate charging voltage. This charging voltage is rectified and filtered to provide the desired DC charging voltage for the battery to be charged.
Regulations governing battery charger efficiencies have been promulgated by various governmental agencies. For example, the California Energy Commission has revised their Appliance Efficiency Regulations to include battery charger circuits. These regulations are set forth in Title 20, Sections 1601-1608 of the California Code of Regulations (“Regulations”). The US Department of Energy has also promulgated standards regarding the efficiency of battery chargers in Title 10, Part 430 of the Code of Federal Regulations.
Unfortunately, many known conventional linear battery charger and conventional high frequency battery chargers are not known to meet the battery charger efficiency benchmarks set forth in the standards mentioned above. Specifically, known linear charger circuits are known to have efficiency in the range from 50% to 75% at full load, which is below the benchmarks set forth in the standards mentioned above. Most of the losses are known to be from the transformer.
In order to address this problem, one known linear charger circuit is known to incorporate a toroidal transformer which has significantly lower losses than bobbin wound transformers. However, there are several drawbacks with respect to the use of toroidal transformers. For example, such toroidal transformers require specialized winding equipment are more labor intensive and have efficiency in the range from 65% to 80% at full load. In addition, as is the case with most known bobbin wound transformers, the efficiency of such toroidal wound transformers is lower at less than 60% of full load. In fact, at 20% of full load, the efficiency of such toroidal wound transformers can be less than 40%.
High frequency charger circuits can be designed to be 80% to 90%+ efficient at full load. However, the efficiency of such high frequency charger circuits is known to be relatively less efficient at less than full load. In addition, high frequency battery chargers are less reliable because of the number of components and the amount of current through those components in an engine start mode.
As mentioned above, the efficiencies of the linear and high frequency battery chargers vary as a function of their loading. The regulations set forth above relate to overall efficiencies. This means that the battery charger must meet the efficiency benchmarks during all conditions in which the battery charger is connected to a 120 volt AC power supply. For example, the California regulations specify that the efficiency benchmark must be maintained over a 24 hour period during the following modes of operation:                A mode when the battery charger is charging a battery.        A mode in which the battery charger is providing a trickle charge to the battery.        A mode in which the battery is disconnected from the battery with the battery charger still connected to the 120 volt AC power supply.        
Moreover, even though the conventional linear and high frequency battery chargers mentioned above may meet the benchmarks specified in the above mentioned regulations during certain operating conditions, such as full load, the efficiencies of such chargers are below the specified efficiency benchmark at operating conditions other than full load. Thus, there is a need for a battery charger circuit that can meet the efficiency benchmarks set forth in the above mentioned regulations.